The present invention relates to doppler radar apparatus for use in measuring the velocity of vehicles such as a farm tractors and the like.
Various operations performed by agricultural equipment must be controlled as a function of the speed at which the equipment is travelling. For example, contemporary agriculture often requires the distribution of liquid or solid herbicides, pesticides, fertilizers, etc. over the area in which crops are or will be planted. If the quantity of liquid or solid applied per unit area is inexact or incorrect, it can decrease the effectiveness of the material being distributed or increase the cost of distribution. A similar distribution control problem arises during planting, in which the spacing between adjacent seeds also affects the cost of the planting and the maximum crop yield. The accuracy of the distribution of seed and other materials per unit area depends upon the accuracy with which (a) the materials are dispensed and (b) the speed of the vehicle can be determined.
The speed of farm tractors and other off-highway equipment is not easily determined with accuracy. The conventional method of measuring the speed of a vehicle, i.e., by measuring the rate of revolution of the wheels which drive the vehicle, is not accurate when applied to farm tractors, for example, due to the high rate of slip of the driven wheels relative to the ground. Measuring the speed of a tractor by measuring the rate of rotation of the tractor's undriven wheels is also inaccurate because the wheels tend to skid during turning and to lift off the ground under certain circumstances.
It has been recognized that the speed of land vehicles may be measured using doppler radar equipment. Doppler radar operates by broadcasting a radio frequency (RF) electromagnetic wave in a thin beam, and measuring the frequency of the wave reflected from the ground relative to the frequency of the broadcast wave. The difference between the two frequencies is directly proportional to the speed of the vehicle.
The doppler radar apparatus should ideally produce a narrow radar beam with substantially no side lobes, so that the beam can be pointed at a defined area of the ground and will not strike and be reflected from adjacent structures, such as vehicle tires. The radiation pattern of the electromagnetic wave generated by the doppler radar apparatus is dependent upon the characteristics of the antenna used with the apparatus. One type of antenna known to have low levels of side lobes is the so-called "dual mode" horn antenna. Dual mode horn antennas are described in the P. D. Potter article entitled "A New Horn Antenna With Suppressed Side Lobe and Equal Beam Widths", the microwave journal, pages 71-78 (June, 1963).
Dual mode horn antennas are designed so that the electromagnetic wave propagates through the horn in two modes. The radiation pattern of the antenna is a composite of the patterns produced by the two modes, and includes substantially no side lobes since the side lobes produced by one mode cancel the side lobes produced by the other mode. The composite pattern thus produced has essentially no radiation energy outside of the main or axial lobe. In the past, it has been presumed that in order to produce the proper boundary conditions for the two modes at the mouth of a dual mode horn, the horn had to be designed to have a rather small flare angle, on the order of 12.5.degree.. A small flare angle results in a relatively long horn, however, since the length of a horn is established by its flare angle. Specifically, the diameter of the horn mouth is essentially determined in accordance with the desired gain and beam width of the resulting RF pattern. Given the preferred horn diameter of four to five inches for land vehicle applications using frequencies in the 24 GHz range, a horn must be nearly two feet long in order to have a 12.5.degree. flare angle. A two foot long horn is simply too large to be of practical use on farm tractors and other off-road vehicles.
Even if a horn having the required radiation pattern and physical size requirements could be designed, problems would still be encountered in making the doppler radar system mechanically durable and reliable. The horn must be capable of withstanding the severe mechanical shocks to which a farm tractor or other off-road vehicle will be subjected. Some materials normally used in electromagnetic antennas or their components are not entirely suitable for use in off-road vehicles. The dielectric lenses sometimes used on horn antennas, for example, are generally constructed of pure polymeric materials. Such materials are brittle and somewhat weak, and would therefore be subject to breakage if used on a farm tractor or other off-road vehicle. Other dielectrics can be substituted for the polyethylenes normally used in RF lens structures. Any dielectric material that is used, however, must generally conform to certain standards of homogeniety, since inhomogeneousness of the material may result in dispersion or polarization of the RF wave which is being focused by the lens.
Furthermore, if more than one type of metal is used in the doppler apparatus, differences in the responses of the metals to temperature changes can cause stressing or loosening of internal components of the apparatus. For example, the horn and its related microwave components are generally formed of aluminum, since aluminum is easily cast into the complex shapes in which the components are to be formed. The exterior housing and horn mounting components, on the other hand, should preferably be formed of steel since steel is inexpensive and rugged. When an aluminum horn assembly is mounted within a steel structure, however, differences between the temperature coefficients of the two materials can cause stressing and loosening of joints between components, thereby degrading the durability or life of the apparatus.
Another problem of the typical doppler radar system relates to the observed tendency of doppler radar systems to indicate that a vehicle is moving when it is in fact stopped. One method of avoiding such erroneous indications would be to disable the speed indication provided by the doppler radar whenever the vehicle tires remained stationary for more than a selected time period. Such a solution to the problem complicates the electrical interconnection between the doppler radar system with the rest of the vehicle, however, and is undesirable for that reason. In one doppler radar system for a vehicle, the problem is solved by disabling the speed indication whenever it is below a selected threshold speed. It would be desirable if a doppler radar system could be devised which simply did not provide the erroneous velocity indication in the first place.